4–20mA / 0–10V Analog Scaling Tool

Convert between a sensor’s analog signal and the real measured value, with PLC raw counts.

How it works

The signal and the measured value are tied together by a straight line. If the signal runs from Smin to Smax and the measured value from Vmin to Vmax:

Value from signal: V = Vmin + (S − Smin) ÷ (Smax − Smin) × (Vmax − Vmin)
Signal from value: S = Smin + (V − Vmin) ÷ (Vmax − Vmin) × (Smax − Smin)
Raw count: count = fraction of span × full-scale count

The raw count is what your PLC input card reports before you scale it in the program. A 250 Ω resistor across a 4–20 mA loop turns it into 1–5 V, which is how many voltage-input devices read a current loop.

Worked example

A pressure transmitter outputs 4–20 mA for 0–150 psi. At 12 mA the fraction of span is (12 − 4) ÷ 16 = 0.5, so the pressure is 75 psi. On a 12-bit input card that counts from the signal minimum, the raw count is 0.5 × 4095 = 2048 (rounded). On a card that reads 0–20 mA, 12 mA is 12 ÷ 20 × 4095 = 2457 counts. Going the other way, 100 psi needs 4 + 16 × (100 ÷ 150) = 14.667 mA.

Things to know

Why 4 mA and not 0? A live zero lets you tell a real zero reading (4 mA) from a broken wire or dead transmitter (about 0 mA). Under NAMUR NE43, a signal below 3.6 mA or above 21 mA marks a failure.

Check what your card counts. Some cards give 0 counts at 4 mA, others give 0 at 0 mA, so 4 mA reads as a count above zero. The setting “Counts start at” covers both. Always check your card’s manual for its exact scaling.

Siemens normalized values. Siemens analog inputs report 0 to 27648 across the nominal range. Over-range readings go up to about 32 511 and are not shown here.

Wire it right. A 2-wire transmitter is powered by the loop, so it needs a supply in series. A 4-wire device has its own power, and the signal is only the output. Mixing these up is the most common reason a loop reads 0 mA.

Accuracy. Real transmitters, shunt resistors (use 0.1% parts) and input cards each add error. The straight-line result shown here is the ideal value.

Non-linear sensors. If the sensor is not linear (for example a square-root flow signal from a differential-pressure cell), you need to linearize it first. This tool only does straight-line scaling.

Common questions

What does 12 mA mean on a 4–20 mA loop?

It is the middle of the span: 50%. A 4–20 mA signal spans 16 mA, so 12 mA is 8 ÷ 16 of the way up.

Can I use this for 0–10 V and 1–5 V signals?

Yes. Pick the signal type from the list. The formulas are the same, only the signal minimum and maximum change.

What if my sensor range is reversed?

Enter the larger number as the value at the signal minimum. For example, a level sensor that reads 100% at 4 mA and 0% at 20 mA uses 100 and 0.

Why does the count differ from my PLC?

Check the card’s resolution and which signal level it treats as zero. Some cards add filtering or a fixed offset, and 15-bit cards may use signed counts.

Sources

  • NAMUR NE 43, Standardization of the Signal Level for the Failure Information of Digital Transmitters.
  • IEC 60381-1, Analog signals for process control systems: direct current signals.

Last reviewed: November 2, 2026